In a backlight module or other illumination devices, to increase the illumination angle of a light source thereof, an optical lens is provided in front of the light source. Light through the optical lens is partly reflected, whereby the utilization of the light is low. To increase the light utilization by decreasing the light reflection, a micro structure is formed on a side of the optical lens facing the light source. Such micro structure though can enhance the light utilization, it can reduce the evenness of the light or even increase the chromatic aberration.